Oxygen gas obtained by PSA has been widely utilized in various industrial fields wherein a large amount of oxygen is used continuously, for example, electric furnace steel manufacturing, oxygen aeration for water treatment, bleaching of pulp, smelting furnace manufacturing, ozone generation and the like, and PSA has been generalized increasingly as a process for readily supplying oxygen at low costs.
As a conventional technique for producing an oxygen enriched gas by PSA, there has been predominantly employed a process which is designed to obtain an oxygen enriched gas in a high yield by providing three or four adsorbers and repeating respective steps for adsorption, recovery, desorption, pressurization, etc. in turn. Then, although a request has been made to obtain a definite amount of an oxygen enriched gas by using a less amount of an adsorbent with a simplified apparatus, it is very difficult to fulfill such a request, practically.
In order to realize such a request, PSA using two adsorbers is proposed. For example, Japanese Patent Kokai No. 59-199503 discloses a process wherein a relatively high yield of an oxygen enriched gas such as about 35% can be attained by incorporating pressure equalization. However, in this process, oxygen concentration in the resulting oxygen enriched product gas is as low as 45%. To the contrary, when oxygen concentration in the product gas is raised up to 90%, the yield is decreased to 20%. Therefore, it has been considered that there is a limit to obtain an oxygen enriched gas having a high oxygen concentration in a high yield by PSA using two adsorbers. That is, in PSA using two adsorbers, enough time can not be spent for pressurization to an adsorption pressure with an oxygen enriched gas in a pressurizing step as a pretreatment of an adsorption step in comparison with PSA using three or four adsorbers. As the result, a raw material mixed gas is fed to an adsorber before the pressure is sufficiently raised in the pressurizing step, which results in a high inflow rate of the raw material gas. Therefore, nitrogen gas is not sufficiently adsorbed by an adsorbent and adsorption break-through of nitrogen gas is liable to occur. In order to prevent this, as disclosed in the above Japanese Patent Kokai No. 59-199503, there is employed a method such that a feeding rate of a raw material gas is limited to 0.5 to 1.5 Nm.sup.3 /H per 1 kg of an adsorbent. However, as far as pressurization is effected only by a raw material gas, the problem of adsorption break-through of nitrogen gas is unavoidable.